Effect of Magnetic Field on the Formation of Radiatively Inefficient Accretion Flow around Black Holes

TL;DR

This study uses MHD simulations to explore how magnetic fields influence the formation of radiatively inefficient accretion flows and thin disks around black holes, highlighting the role of magnetic diffusivity and plasma beta.

Contribution

It introduces a new condition involving an equivalent timescale to determine when a thin disk forms in magnetized accretion flows.

Findings

Thin disks form when plasma beta exceeds 600-800.

Persistent jets and outflows are observed from the corona.

The formation depends on the ratio of cooling time to an equivalent heating timescale.

Abstract

We study the effects of magnetic field in the formation of a radiatively inefficient accretion flow (RIAF) in the presence of Bremsstrahlung cooling, which facilitates the formation of a geometrically thin, optically thick accretion disk surrounded by a hot corona. We have performed axis-symmetric magnetohydrodynamic (MHD) simulations of an initial accretion torus with a $1/r$ dependant local poloidal field in the presence of a pseudo-Newtonian potential, taking into account optically thin cooling, resistivity and viscosity. We observe the formation of persistent jets and magnetised outflows from the corona surrounding a thin disk with an increase in the magnetic diffusivity parameter. We have defined an equivalent time scale ($\tau_{eq}$) which takes into account the heating time scales due to viscosity, resistivity, magnetic reconnection and magneto-rotational instability turbulence such that the thin disk is formed if the cooling time scale ($\tau_{cool}$) is lower than this equivalent time scale ($\tau_{cool}/\tau_{eq}<1$). Using this condition, for the first time, we found that the thin disk exists when the initial ratio of plasma pressure to magnetic pressure (plasma beta) exceeds a range of $600-800$ for the gas obeying a polytropic equation of state accreting at $10^{-5}\ M_{\odot}/year$